Birds of a feather eat together (sometimes)
Researcher finds surprising social behavior of healthy birds and their sick flock-mates.
By Catherine Doss, College of Science
Related story: Fellowships support wildlife disease research
As part of her research into the transmission of diseases among wild animals, Dana Hawley, assistant professor of biological sciences, made a surprising discovery. House finches (Carpodacus mexicanus), especially males, tended to feed near flock mates who were visibly sick.
“Instead of finding that they avoided the sick bird, we found the opposite,” Hawley says.
Her study of the spread of the naturally occurring bacterial pathogen Mycoplasma gallisepticum among finches in captivity was part of the broader scope of her research on the ecology and evolution of wildlife diseases.
“It fascinates me that even though finches live in a closely linked group, some animals get sick and others don’t,” she says. “I’m trying to understand why this happens from a behavioral perspective.”
Hawley and her post-doctoral research associate Karen Bouwman studied house finches in a caged and controlled environment over the course of a year. During the experiment, healthy finches were given two feeder choices to share with another finch. One feeder was occupied by a healthy bird; the other by a sick one. The birds could see but not contact each other. Females were studied with other females; males with other males.
She began by asking whether healthy birds actively avoid feeding near contagious flock mates.
M. gallisepticum is a contagious illness that causes lethargy and symptoms resembling pinkeye. It is a relatively new pathogen among finches, first appearing in 1994. Disease symptoms are clearly visible.
“I wanted to see if birds recognized sick birds and if they avoided them,” Hawley says. “Since humans are able to see the symptoms, I wanted to see if birds could see them, too, and how this would affect the spread of the pathogen in natural populations.”
Finches were studied because of their adaptability and the fact that in the eastern United States they feed almost exclusively from bird feeders. Most of the interaction between finches takes place at the food source, which makes finches relatively easy to study.
What they found
Somewhat to their surprise, Hawley and her research team found that healthy birds chose to eat with sick birds rather than fellow healthy birds 75 percent of the time. Male birds were especially likely to make this choice.
"We know almost nothing about how animals respond to infected members of the same species."
The reason for this, they believe, is that a bird was less likely to be the victim of aggression from the other bird at the feeder if that bird was sick. Aggression levels were observed to be lower among sick birds. Past studies by Hawley showed that aggression at the feeder (a common occurrence) is costly to a finch’s health.
“So the birds we studied were probably trying to avoid an aggressive interaction by gravitating to a sick bird, but by doing so, they were more likely to get infected,” Hawley says.
Aggression was measured by how many times one bird kicked another bird off the feeder. If one bird was socially dominant, it would displace the other bird from feeding. The same holds true for finches in the wild. Another interesting finding was that it was males, not females, that preferred to associate with sick birds of the same sex at the feeder. Healthy males were much more likely to win aggressive encounters with infected males.
But females showed no such competitive edge when confronting sick females. The reasons for this remain unknown, but when sick, many male animals show lower levels of testosterone, a hormone important for aggression. Hawley is currently exploring whether this may play a role in the differences between male and female finches. She says that the overall findings of attraction to sick birds were surprising, but made sense given the fact that birds want to stay away from aggressive neighbors.
“That’s the thing about science,” she says. “You don’t necessarily connect the dots until the study is over.”
The broader perspective
Ultimately, Hawley hopes to learn whether this seemingly self-sabotaging behavior will reverse itself the longer the disease circulates in the population. Every winter, there is an epidemic of M. gallisepticum among free-living finches. Usually animals develop some kind of resistance over time. But Hawley thinks their tendency to not keep their distance from fellow sick birds is causing the transmission to be augmented in finches every year. “It’s a unique way of maintaining a cycle of a pathogen epidemic,” she says.
Hawley’s research is funded by a grant from the National Science Foundation as part of a collaborative program on the ecology of infectious diseases. Results from her study were published in the journal Biology Letters and featured in the journal Nature.
"We’d like to find a way to break the cycle of transmission in order to manage particularly devastating diseases in natural populations."
“Pathogens are a big problem in wildlife conservation,” she says. “So we want to learn why pathogens spread through some populations and not others. We’d like to find a way to break the cycle of transmission in order to manage particularly devastating diseases in natural populations. Hopefully what we learn will be valuable in future conservation decision-making.”
Eventually this knowledge will lead to predictive power not just in birds, but other animals as well. Hawley expects more studies like this one in the future.
“We know almost nothing about how animals respond to infected members of the same species,” she says. “There are so many questions we haven’t begun to look at.”
Perhaps with continued exposure to this disease, finches will catch on to the contagious dangers their dinner mates are carrying.
“When we look at a bird, we know it’s sick and instinctively know to stay away,” Hawley says. “It’s so easy for us to see; how come they’re not figuring it out?”